Semiconductors are widely used in integrated circuits for electronic applications, including radios and televisions. Such integrated circuits typically use multiple transistors fabricated in single crystal silicon. Many integrated circuits now contain multiple levels of metallization for interconnections. A single semiconductor microchip ;may have thousands, and even millions of transistors. Logically, a single microchip may also have millions of fines interconnecting the transistors.
While semiconductor devices are being scaled in the horizontal dimension to reduce wafer cost by getting more chips per wafer or to increase circuit complexity by getting more transistors per chip, they are not necessarily being scaled in the vertical dimensions. One of the reasons for this is that as metals are scaled in the horizontal dimension, the current density of the leads increases. If the metal leads were also scaled in the vertical dimension, the current density would exceed reliability limits. With horizontal scaling, these tall metal leads are being packed closer and closer together causing capacitive coupling between the leads to become the primary limitation to circuit speed.
The material typically used to isolate metal lines from each other is silicon dioxide. Silicon dioxide is a thermally and chemically stable material. Conventional oxide etches are also available for high-aspect-ratio contacts and vias. However, the dielectric constant of dense silicon oxide grown by thermal oxidation or chemical vapor deposition is on the order of 3.9. The dielectric constant is based on a scale where 1.0 represents the dielectric constant of a vacuum. Various materials exhibit dielectric constants from very near 1.0 to values in the hundreds. As used herein, the term low dielectric will refer to a material with a dielectric constant less than 3.3.